Optical transmission articles each with refractive indexes gradually distributed from the center toward the periphery on a cross-section thereof are disclosed in Japanese Examined Patent Publication No. 47-816, Japanese Examined Patent Publication No. 47-28059, and European Patent Publication No. 0208159.
The distributed refractive index type optical transmission article disclosed in Japanese Examined Patent Publication No. 47-816 is made of glass and fabricated by an ion exchange method. This method, however, has a poor productivity and cannot produce articles having an identical shape (an identical length, in particular) and an identical performance. Even with an identical performance, the fabricated distributed refractive index type optical transmission articles have different lengths, and thus problems arise with the handling thereof.
The distributed refractive index type optical transmission plastic article disclosed in Japanese Examined Patent Publication No. 47-28059 is made by mixing two or more transparent polymers having different refractive indexes and different solubilities with respect to a particular solvent. The mixed polymers are shaped into a rod or a fiber, and immersed in the solvent to extract a part of the polymers from the surface thereof, thereby changing a mixing ratio of the polymers from the surface toward the center thereof. Since the optical transmission article of this method is made of a mixture of two or more polymers having different refractive indexes, fluctuations in the refractive indexes of the article occur, deteriorating the transparency and causing light scattering, and therefore, the article cannot serve as the distributed refractive index type optical transmission article. Accordingly, little improvement can be expected in the application and development of this method.
European Patent Publication No. 0208159 describes a method in which (A) at least one kind of thermoplastic polymer and (B) a monomer which is compatible with the polymer (A) when polymerized and forms a polymer having a refractive index different from that of the polymer (A), are mixed and formed into a rod-like shape. From the surface of the shaped body, the monomer (B) is volatilized to continuously distribute the monomer (B) from the surface toward the interior of the formed material, and thereafter, the non-polymerized monomer in the shaped body is polymerized to form a distributed refractive index type optical transmission plastic article.
An ideal refractive index distribution curve of the distributed refractive index type optical transmission article is expressed as follows: EQU N=N.sub.0 (1-ar.sup.2)
This curve is considered to be the same as a curve "a" of FIG. 2.
According to studies and measurements made by the inventor, using an Interfaco interference microscope under conditions to be explained later, however, the distributed refractive index type optical transmission article fabricated according to the above-mentioned method provides a refractive index distribution curve "b" of FIG. 2. In a range of 0.5r.sub.0 to 0.75r.sub.0 of radial distances from the center (i.e., in a range of c to d in the figure, with e being an outermost part), the curve b is relatively close to the ideal curve expressed by the equation (1). At the outer and inner sides of the above range, however, the refractive index distribution is greatly deviated from the ideal curve.
When a grid pattern is observed through these optical transmission articles, if the optical transmission article has a refractive index pattern which almost correctly follows the quadratic curve defined by the equation (1), the article will provide a normal grid image as shown in FIG. 3(a). If, however, the refractive index distribution of the optical transmission article deviates from the ideal refractive index distribution, as indicated by (b) of FIG. 2, the article will provide a distorted grid image as shown in FIGS. 3(b) and 3(c), since the article can not transmit a correct image. In this case, a moderation transfer factor (MTF) indicating the resolution of the article is very low, i.e., less than 30%, which is not acceptable for use as an optical transmission article of a copying machine.
Accordingly, the conventional distributed refractive index type optical transmission article with the refractive index distribution as shown by (b) of FIG. 2 must be cut or eluted by a solvent process to remove a portion outer than the position (d) of FIG. 2, thereby providing the optical transmission article with an optical path having a relatively ideal refractive index distribution. It is, however, difficult to provide an optical transmission article with a high resolution, and thus the productivity thereof is very low, and it is very difficult to constantly produce products having a uniform quality.